hall at charon.ns.utk.edu
Thu Sep 10 14:36:16 EDT 1998
There is an excellent book out called the Evolution of Flight in
Insects, by Andrei Brodsky. Brodsky discusses the aerodynamics of
flapping flight and comparative studies of wings and comes to the
sensible conclusion that wings evolved from the gills of aquatic
insects. I'll try to explain the evidence as briefly as I can.
The primitive protoinsect most closely resembled extant Mayflies,
Ephemeroptera, and was an aquatic critter with externalized gills on its
abominal and probably thoracic segments too. These gills had a
musculature associated with them which allowed the organism to move them
through the water, enhancing oxygen absorption. This allowed the
organism to invade oxygen-poor habitats such as marshes and swamps,
where the first insect appears to have evolved. The protoinsect was
probably marine or tidal and shrimp-like. Many larval Mayflies retain
shrimp-like characters, including locomotion by tail-flipping.
The invasion of brackish marshes probably continued into freshwater and
inland into rivers and streams. The protoinsect likely had swarming,
short-lived reproductive forms as do extant Mayflies, wherein
individuals essentially exerted all of their energy to enter a swarm and
copulate, dying shortly therafter. This placed a premium on motility,
and gills expressed on the thoracic segments, where musculature was
already stronger and better developed, became increasingly useful as
swimming organs. Predation pressure probably drove these mating swarms
out of the water, where thoracic gill flapping became weak flight became
strong flight. Aside from its value as a predator-avoidance mechanism,
flight may have also allowed these early insects to disperse upstream
more easily than they could have in the water. Similarly, during the
invasion of stagnant waters, the protoinsect would become more dependent
on crawling or swimming to get around, whereas its predecessors might
have relied on tides and currents to get around.
That's the basic story as I understand it. There is solid evidence to
back this story up:
1. There is extensive homology between gills and wings, including
venation patterns and muscle attachment sites. Kukolova-Peck has
exhaustively demonstrated this during her career, and Brodsky reviews
her findings in his book.
2. The aerodynamics of flight is such that gliding is more difficult to
achieve than powered flight (flapping), and phylogenetic analysis shows
that gliding has evolved from flapping on numerous occassions, whereas
flapping seems to have evolved once and is the primitive state. Brodsky
provides mathematical and experimental support for this point.
3. The fossil record is consistent with wings evolving in an aquatic
insect in a swamp, as the oldest insect fossils are Mayflies found
amidst other swamp dwellers.
4. Water being more bouyant than air, it would be easier to evolve
flight in water, develop strength, and then conquer the air.
This scenario is vastly superior to the standard tale which still runs
on a video screen in the Ortho Insect Zoo in the Smithsonian and which
is still taught in entomology classes by the majority of entomologists
who haven't kept up to date. The standard tale is that a jumping insect
like a grasshopper evolved flattened protrusions from its thorax which
either allowed it to glide farther on jumps or to thermoregulate its
body. Somehow theses protrusions developed musculature, and presto!
wings. The magical evolution of musculature is unexplained in this
scenario, and it is inconsistent with phylogeny and the fossil record
It is curious that several of the oldest bird fossils are aquatic and
that the extant form considered most primitive is the Loon, an aquatic
diver. I think that it is only a matter of time before ornithologists
adopt a similar tale to explain the evolution of flight in birds, with
wings evolving as diving/swimming organs before becoming capable of
Spread the word, entomologists, and check out Brodsky's book.
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